In general, aluminum anodes for use in electrolytic capacitors are formed by electrolytically etching aluminum foils and then electroforming the etched foils to form oxide coatings on the surfaces. It has been found that in order to obtain electrolytic capacitors of low leakage current it is necessary to employ aluminum foils of high purity, generally of a purity of greater than 99.96%. Further it has been found that the main impurity influencing the leakage current is the iron content in the foil. As a result, it has been further established that satisfactory capacitors are obtained only by use of aluminum foil containing not more than 0.010% by weight of iron.
However, the use of aluminum foils of such high purity has proved to be expensive and thus it is desirable to employ a less pure and therefore a less expensive aluminum foil.
A method employed in the art that enables the use of aluminum foil of low purity is shown in Japanese Patent Application No. 1011/78 filed Jan. 31, 1978 and open for inspection on August 1979. In this patent the use of aluminum foil of an aluminum content as low as 99.2% and an iron content as high as 0.33% is disclosed.
The method disclosed in this patent comprises first heat treating the foil at 450.degree.-600.degree. C. for a period of time of 10 to 60 minutes then dipping the foil in a solution of 2-20% phosphoric acid 60.degree. C. for about 5-10 minutes and then dipping the foil into a 2-20% nitric acid solution temperature of 60.degree. C. for about 2-20 minutes.
In Example II of this patent the application of this method to an aluminum foil of 99.2% purity and an iron content of 0.33% is shown to result in a leakage current decrease to 35 .mu.A from 350 .mu.A.
Further, according to this patent, the function of the heating treatment is to deposit the iron impurities on the interfaces of the aluminum crystallite granules. The function of the aqueous phosphoric acid is to remove the iron impurities from the intergranular boundaries not from the surface aluminum foil. However, at the same time, a partially insoluble aluminum phosphate is formed on the surface of the aluminum foil. This may cause difficulties in the forming process. The phosphoric acid treated foil is therefore dipped into the nitric acid solution which acts to remove the aluminum phosphate from the surface of the foil.
However, the use of both nitric acid and phosphoric acid has proved to be costly both in regards to the cost of materials and in regard to the necessary expenditure of time. Further the use of the phosphoric acid results in the need for expensive waste control facilities.